One of the most effective means of accelerating charged particles to relativistic speeds is the linear accelerator (linac). A linac is made of a series of resonant cavities. As a packet or bunch of particles, such as electrons, pass through each cavity in a linac, an intense electric field provides an acceleration gradient for the particles in order to increase their kinetic energy. With reference to FIG. 1, conventional cavities 100 for linacs utilize a cavity geometry that is surface of revolution having a cross section 102 along the axis 108. The cavities are lined with a conductive material, so that when electromagnetic wave energy is supplied to the cavities, the cavities will support one or more resonant standing wave patterns. The fundamental harmonic of electric 106 and magnetic 104 fields in each cavity 100 will provide the desired acceleration gradient along the central axis. This fundamental harmonic is referred to as the accelerating mode.
In the accelerating mode, also conventionally called the TM100 mode, the electric field 106 is at a maximum at the central axis 108 of the cavity 100 and is directed parallel to the axis. This electric field 106 applies an accelerating force on the bunch of particles passing through the cavity. Moving farther away from the axis, the electric field 106 is curved toward the surface of the cavity 100 in order to satisfy boundary conditions with the surface of the cavity. At the equator (i.e., the largest radial extent from the central axis) of the cavity 112 the electric field 106 is at a minimum. At the iris (i.e., the smallest radial extent from the central axis) 114 the electric field applied to the cavity is the greatest since it is the closest point to the axis 108.
The magnetic field 104 and the electric field 106 are related according to the right-hand-rule. As shown in FIG. 1 the magnetic field is directed into the page at the top of the cavity 100 and out of the page at the bottom of the cavity 100. The magnetic field 104 is at a maximum at the equator of the cavity 112 and is at a minimum at the axis of the cavity 108. The value of the maximum magnetic field 104 is directly proportional to the maximum surface current density on the cavity shell. Also, the maximum magnetic field 104 at the equator of the cavity 112 strongly determines the maximum acceleration gradient of the cavity 100. Therefore the maximum surface current density of the cavity shell strongly determines the maximum acceleration gradient. Since this is the case, superconducting materials may be preferred as a cavity lining to enable a high surface current density and correspondingly a high acceleration gradient.
FIG. 2 shows a cross-section taken through the equator of the cavity 100. This cross-section shows a first harmonic wave pattern of the cavity structure. This first harmonic is detrimental to particle acceleration since it generates an electric field 202 that is transverse to the axis 108, causing deflection of the particle bunches passing through the linac. The surface currents 206 are determinant in the magnitude of the electric field for causing the deflection. These surface currents travel in an azimuthally oriented direction around the equator of the cavity 112. Various other higher order mode harmonics similarly create transverse effects to the particle bunches and are collectively referred to as deflecting modes or higher order modes (HOM).
The deflection of the particle bunches causes the dilution of the brightness of the particle beam, head to tail instabilities, multi-bunch coupling instabilities, etc. The harmonics of the deflecting modes are caused by misalignment of cavities or strings of cavities. FIG. 3 depicts multi-bunch coupling with the transverse electric field growth driven by deflecting modes that are excited when a bunch is displaced off-axis in the cavity. FIG. 4 depicts the head-to-tail instabilities caused when a bunch passes through a misaligned string of cavities. It is noted that in conventional cavity designs, these deflecting modes gain all of the advantages of the resonance that the fundamental mode has, and therefore generate a Q in the same order of magnitude as the Q for the fundamental harmonic.
The International Linear Collider (ILC) project has been endorsed as the next new facility for high energy research. The Teraelectronvolt Energy Superconducting Linear Accelerator (TESLA) technology has been chosen for the ILC project as the most cost-effective basis for the ˜500 GeV linear accelerators (linacs). A second major project, the X-ray Free Electron Laser (XFEL) at the Deuches Electronen Synchrotron (DESY) in Hamburg, Germany, also utilizes the TESLA cavity structure. In both projects, the capital cost of a TESLA-based linac will be dominated by the cost of the Nb cavities and the associated cryogenics, power couplers, and radio frequency (RF) power systems. The operating cost of a TESLA-based linac will be dominated by the cost of refrigerating kilometers of accelerating structure to superfluid helium temperature.
The performance of a linac is determined by the accelerating gradient that can be sustained and by the beam brightness (emittance density) that can be sustained through the acceleration process. Dilution of the beam brightness can arise from instabilities in the particle motion through the linac and from the transverse forces due to deflecting modes in the superconducting cavities as was described above. It would be desirable to have an improved cavity design that intrinsically suppresses undesired deflection modes while simultaneously enabling the creation of cavity linings having substantially higher maximum surface current densities. Moreover, it would be desirable for such a cavity design to enable operation with more efficient refrigeration, thereby reducing capital and operating costs of high power linacs.